Blood clotting is one of the most important and complicated physiological methods of protecting the human body. There are two major systems involved in blood clot formation. These are blood platelets and the thrombin system. Platelets are tiny cellular elements which are formed in the bone marrow, where they are derived from megakaryocytes. They are extremely small and measure about 1- 3 um in size (micrometers). Platelets leave the bone marrow and circulate for only a short time in the blood, for approximately ten days before they die and are sent to the spleen.
When a blood vessel is broken (for example when a cut occurs) collagen is released at the site of injury. Platelets are able to recognize collagen and von Willebrand factor and they then step in to try and stop the bleeding. Once platelets start to reach the wound area, they send out signals to other platelets to come to the area. This is known as platelet aggregation. By doing this they become "sticky" and they hold fast to the blood vessel wall and each other, forming their own mini clot to stop the bleeding. This will stop further damage to the injured blood vessel. The normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. Counts which are much higher than this increase the risk of blood clot and stroke, whereas low counts risk severe bleeding.
In addition to the presence of blood platelets, blood clotting is helped along by a group of fibrin molecules, which are insoluble and help to hold the clot in place. A group of complex reactions involving fibrinogen, thrombin, tissue factor and factors 7 and 10 all need to be working correctly in order to lead to the production of the fibrin that is necessary to help stop bleeding. Fibrin is stringlike and, in the presence of bleeding, many fibrin "strings" together help to form the clot. Together with platelets, these fibrin strings help to stabilize the clot formed by binding the platelets together, so that the clot does not break up.
Depending on the area of bleeding, the mechanism for blood clotting is different. Platelets tend to aggregate when bleeding occurs in arteries. When bleeding occurs in veins, the main method of clotting is by the thrombin system. Various other tissue factors are involved in helping to make sure that the formation of a blood clot works accurately, including those that dissolve the clot once the bleeding has stopped. They are necessary to make sure there is never excessive clotting or bleeding.
This is just an overview of the mechanisms of blood clotting, as it is a very complicated and technical procedure.